Modern firearms are designed and manufactured to operate with multiple interoperational components and often with modular construction. In one example, an AR-10 or AR-15 style sporting rifle 100 uses a modular construction with an upper receiver 102 and lower receiver 104, as shown in FIG. 1a. Lower receiver 104 is characterized by trigger guard 106, trigger assembly with hammer 108, pistol grip 110, and magazine well 112. Buttstock 114 attaches to lower receiver 104. Upper receiver 102 is characterized by bolt carrier assembly, forward assist, charging handle, and gas-operated reloader. Barrel assembly 120 with handguard 122 attaches to upper receiver 102. Lower receiver 104 is attached to upper receiver 102 by removable rear take-down pin 124 and forward pivot pin 126. Removing rear take-down pin 124 allows upper receiver 102 to hinge and rotate about forward pivot pin 126, see FIG. 1b. 
The AR-15 platform is designed to have clearance fit/free running between lower receiver 104 and upper receiver 102. The standard tolerances between upper receiver 102 and lower receiver 104 causes the upper and lower receiver to not fully interlock. When AR-15 100 is fully assembled with upper receiver 102 engaging lower receiver 104 and pins 124-126 in place, as in FIG. 1a, there remains some gap or space between the upper receiver and lower receiver, given design and machining tolerances. In other words, upper receiver 102 and lower receiver 104 engage with a loose fit. The tolerances are necessary for free running assembly and dis-assembly of the firearm. If the tolerances are too tight, the lower receiver and the upper receiver will jam, improper fit, or difficulty with assembly and disassembly. In addition, the space leads to movement of upper receiver 102 relative to lower receiver 104 during firing, malfunction of the firearm, cartridge jams, and reduced targeting accuracy. Some manufacturers may have loose tolerances and low manufacturing quality control with respect to fitment between upper receiver 102 and lower receiver 104. The jamming and malfunction can lead to reliability and safety problems.